Having troops in Iraq would not be necessary had a massive, US lead effort to destabilize Syria not been executed.
More importantly, Iraq demanded that we leave. If we had tried to stay, we would have been attacked by the entire Arab population. It would have been a disaster. Leaving was inevitable.
Anti-American Iran (still chanting “Death to America” after all these years) is a big reason why US troops in Iraq would be needed in any case - well helpful anyway. But the big reason is that the Americans were the best honest brokers available to stabilize Iraq, after a generation that knew only brutal dictatorship.
Iraq only failed to sign a Status Of Forces Agreement (SOFA) with the US, because they were not seriously negotiated with. They always drive every negotiation past the final deadline - carpet-dealing style hard driving haggling is just part of the National character. The Americans just suddenly said “OK”, and walked away.
Bottom Line: The Obama Admin pretended to lose the negotiation, because they wanted to pull out. They announced a withdrawal date, and rebuffed overtures to re-engage on negotiations.
The Iraqis saw they were going to be abandoned to Iran’s tender mercies, so few would speak out in favor of a SOFA after that. Many in Iraq wanted the Americans to stay.
The joke was that the Kurds say they wanted the Americans to stay, and they meant it. The Sunnis would say they wanted the Americans to go (being PC), but they did not mean it (they feared Shia and Iranian abuse - which occurred). The Shia would say that they want the Americans to stay (grateful for liberating them from Saddam), but they really didn’t (they wanted the freedom to run roughshod over the others). There is a lot of truth to that joke, but a lot of the Middle class and wealthy Shia really wanted the Americans to stay too, because they saw the alternative as corrupt and religiously extreme Iranian infiltration/domination (which is occurring). Many with family members overseas, or patriotic returning ex-pats recognized that American mentorship gave Iraq a chance at a modern Western-style society/Government, and wanted that.
Radical Shi’ites who led groups of street thugs from the slums (like Muqtada al Sadr), and those militia leaders with financial, military, intelligence, and assassin backing from the Iranian regime have a disproportionate voice in Shi’ite politics in Iraq, and try to silence other voices by any means. A strong American presence (and little else) could have kept a check on that, but Obama decided to throw them all to the dogs (after releasing a flood of all the worst killers and ringleaders onto the streets from the Camp Bucca Confinement Facility).
Iraq had stabilized and was rebuilding. Obama scuttled it, and sent it back into its worst sectarian violence, and the worst physical destruction that Iraq has endured in the modern era - far worse than “Shock and Awe”. Whole cities have been mostly destroyed in the Sunni/Shia civil war with ISIS, ancient Christian and Yezidi communities were devastated, and graveyards filled. Those who supported America, and Western values, have been the hardest hit - often systematically murdered in great detail.
The “Arab Spring” Obama policies of supporting Muslim Brotherhood takeovers across the Middle East and North Africa, including covert support of jihadis in Syria, as well as abandoning or Iraqi allies has been the great disaster of the 21st century to date.
Obama’s abandonment of Iraq was even more disastrous for them, than the abandonment of Vietnam was for the South Vietnamese - and that is saying a lot.